The first generation of personal wireless communication devices, such as cellular radio telephones, operated by allocating distinct individual radio carrier frequencies to each user. For example, in an Advanced Mobile Phone Service (AMPS) type cellular mobile telephone, two 30 kiloHertz (khz) bandwidth channels are allocated to support full duplex audio communication between each subscriber unit and a base station. The signals within each such channel are modulated using analog techniques such as Frequency Modulation (FM).
Later generation systems make use of digital modulation techniques in order to allow multiple users to access the same frequency spectrum at the same time. These techniques ostensibly increase system capacity for a given available radio bandwidth. The technique which has emerged as the most popular within the United States is a type of Code Division Multiple Access (CDMA). With CDMA, each traffic signal is first encoded with the pseudorandom (PN) code sequence at the transmitter. The receivers include equipment to perform a PN decoding function in such a way that signals encoded with different PN code sequences or with different code phases can be separated from one another. Because PN codes in and of themselves do not provide perfect separation of the channels, certain systems have an additional layer of coding referred to as “orthogonal codes” in order to reduce interference between channels.
In order for the PN and orthogonal code properties to operate properly at a receiver, certain other design considerations must be taken into account. For signals traveling in a reverse link direction, that is, from a mobile unit back to a central base station, power levels must be carefully controlled. In particular, the orthogonal properties of the codes are optimized for the situation where individual signals arrive at the receiver with approximately the same power level. If they do not, channel interference increases. It has been possible in the past to set power levels individually to optimize each channel, by for example, adjusting it to affect an optimum received power level at the base station.
Newer generation systems also make use of coding algorithms such as forward error correction (FEC) type algorithms based upon convolutional, Reed-Solomon, or other types of codes. Such FEC codes can be used to increase effective signal-to-noise ratio at the receiver. While such codes do provide increased performance in terms of lower bit error rates in noisy environments, by themselves they do not improve the difficulties associated with co-channel interference. Furthermore, the introduction of the possibility that a given field unit might be using a different FEC coding rate than another unit exacerbates design decisions with respect to prudent power management from the perspective of the system as a whole.